1. Field of the Invention
This invention relates to non-invasive approaches for determining cardiac output in patients, specifically to partial re-breathing techniques for determining cardiac output in patients, and most particularly to devices for storing and subsequently re-introducing into the ventilator circuit a volume of expired air in order to accomplish re-breathing, as well as ventilator circuits so equipped.
2. Statement of the Art
It is desirable, or even essential, to determine or monitor the cardiac output of a patient in many medical and surgical procedures. Invasive techniques well known and used in the art employ the use of catheters inserted at certain arterial points (e.g., femoral artery, jugular vein, etc.) to monitor blood temperature and pressure in order to determine cardiac output of the patient. Although capable of producing reasonably accurate results, the invasive nature of such procedures, with the attendant trauma and risk of infection, has demonstrated an unreasonably high potential for morbidity and mortality consequences.
Adolph Fick""s measurement of cardiac output, first proposed in 1870, has served as the standard by which all other means of determining cardiac output have been evaluated since that date. Fick""s well-known equation, written for CO2, is:   Q  =            V              CO        2                    (                        C                      V                          CO              2                                      -                  C                      A                          CO              2                                          )      
where Q is cardiac output, VCO2 is the amount of CO2 excreted by the lungs and CACO2 and CVCO2 are the arterial and venous CO2 concentrations, respectively. Notably, the CO2 Fick Equation usually presumes an invasive method (i.e., catheterization) of determining cardiac output because the arterial and mixed venous blood must be sampled in order to determine arterial and venous CO2 concentrations.
It has previously been shown, however, that non-invasive techniques may be used for determining cardiac output while still using principles embodied in the Fick Equation. That is, expired CO2 (xe2x80x9cpCO2xe2x80x9d) levels can be monitored to estimate arterial CO2 concentrations and a varied form of the Fick Equation can be applied to evaluate observed changes in pCO2 to estimate cardiac output. One use of the Fick Equation to determine cardiac output in non-invasive procedures requires the comparison of a xe2x80x9cstandardxe2x80x9d ventilation event to a sudden change in ventilation which causes a change in expired CO2 values and a change in excreted volume of CO2. One commonly practiced means of providing a sudden change in effective ventilation is to cause the ventilated patient to re-breath a specified amount of previously exhaled air. This technique has commonly been called xe2x80x9cre-breathing.xe2x80x9d
Prior methods of re-breathing have used the partial pressure of end-tidal CO2 to approximate arterial CO2 while the lungs act as a tonometer to measure venous CO2. Such an approach to re-breathing has not proven to be satisfactory for determining cardiac output because the patient is required to breath directly into and from a closed volume in order to produce the necessary effect. However, it is usually very difficult for sedated or unconscious patients to actively participate in inhaling and exhaling into a bag. The work of some researchers has demonstrated that the Fick Equation could be further modified to eliminate the need to directly calculate venous PCO2 (PVCO2) by assuming that the PVCO2 does not change within the time period of the perturbation- an assumption that could be made by employing the partial re-breathing method. (See, Capek et al., xe2x80x9cNoninvasive Measurement of Cardiac Output Using Partial CO2 Rebreathingxe2x80x9d, IEEE Transactions On Biomedical Engineering, Vol. 35, No. 9, September 1988, pp. 653-661.)
Known partial re-breathing methods are advantageous over invasive measuring techniques because they 1) are non-invasive, 2) use the accepted Fick principle of calculation, 3) are easily automated, 4) require no patient cooperation and 5) allow cardiac output to be calculated from commonly monitored clinical signals. Thus, non-invasive cardiac output techniques are rapidly gaining favor. However, most known re-breathing circuits for storing expired air and then delivering it to the patient for partial re-breathing cause an increase in the volume and resistance of the respiratory path, which may complicate the operation of the ventilator. In addition, many conventional re-breathing circuits provide only a fixed re-breathing volume, which may not be optimum, or even suitable, for patients of various sizes and respiratory capacities. Finally, conventional re-breathing circuits frequently include components which are of complex and relatively expensive construction, making them contamination-prone, difficult to sterilize, and too expensive to be used as disposable units.
It would be advantageous to provide a re-breathing circuit which accomplishes re-breathing with little or no change to the respiratory path volume or air flow resistance, to minimize or eliminate interference with the ventilator function and reduce the load xe2x80x9cseenxe2x80x9d by the patient. It would also be advantageous to provide a re-breathing circuit in which re-breathing volumes can be varied as needed. It would be desirable for the re-breathing circuit to be usable with state-of-the art ventilator circuits and monitors without modification thereto. In many cases it is desirable for a re-breathing circuit to be made up of relatively simple and inexpensive, easy to fabricate, one-use (disposable) components.
The present invention is a novel re-breathing apparatus suitable for use with a mechanically ventilated patient. The re-breathing apparatus includes a dual-chamber gas reservoir, each chamber of which is connected via tubing to a diverting adapter which allows flow of air to and from a respiration circuit. The total volume of the reservoir is fixed and the two chambers operate in reciprocal fashion, with one expanding to draw in air while the other is contracting to expel air. Accordingly, the total volume of gases in the reservoir, as well as in the respiration circuit as whole, can remain unchanged during operation of the apparatus in a re-breathing mode. The invented re-breathing apparatus thus has the advantage that it is entirely xe2x80x9cinvisiblexe2x80x9d in terms of operation of the respiration circuit, because there is no change in either the air volume or resistance to flow of the circuit (a factor which is of particular concern when a ventilator is used in the breathing circuit). A method of operating the re-breathing apparatus, the re-breathing apparatus and its components, and a respiration circuit including the inventive re-breathing apparatus are encompassed by the scope of the invention.